Tho height to which a balloon will rise is determined from the law according to which the density of the atmospheric strata diminishes as the distance from the earth is increased. The buoyant force diminishes with the density ; and when it is reduced to a quantity only equal to the weight of the balloon and its appen •dages, no further ascension can take place. Another circumstance also con fines the possible elevation within moder ate limits. As the pressure of the exter nal air is diminished, the expansive force of the confined gas becomes greater, and would ultimately overcome the resistance of any material of which a balloon can be made. A balloon quite filled at the sur face of the earth would inevitably be torn to shreds at the height of a few miles in the atmosphere, unless a portion of the gas were allowed to escape. For this pur pose the balloon is furnished with a safety valve, which can be opened and shut at pleasure ; but to prevent unneces sary waste of gas, it ought to be made of such a size that it requires only to be partly filled. A balloon half filled at the surface of the earth would become fully distended at the height of 31 miles.
We have hitherto spoken only of bal loons filled with hydrogen gas ; but it is evident that any other substance specific ally lighter than air would answer the purpose ; in fact, the first balloons by which any one was raised into the atmos phere were not filled with hydrogen, but simply with rarefied air, the rarefaction being produced by kindling a fire under them ; and as they thus became filled with smoke, they were called smoke-bal., loons. The ascensional force, however, which can be gained in this way is not great ; besides the aeronaut must carry a portion of fuel with him for the purpose of maintaining the fire, which adds sensi• bly to the weight to be raised. The keep ing up of the fire is also attended with inconvenience, and even danger.
Two brothers, Stephen and Joseph Montgolfier, proprietors of a paper manu factory at Annonay in France, have the honour of first preparing and sending up a balloon into the air. After one or two previous trials, they announced a public ascent on the 5th of June, 1788. The balloon was prepared of linen cloth ; a fire was kindled under it, and fed with bundles of chopped straw. This sub stance was used with a view to produce a large quantity of smoke. It would seem that they attributed the elevation of the balloon to the ascending power of the smoke, instead of its true -cause, the rare faction of the heated air. In the space of five minutes it was completely dis tended; and on being let slip, ascended rapidly. It reached an elevation of about
a mile, remained suspended ten minutes, and fell at the distance of a mile and a half from the place of its ascension. When the news of this experiment was carried to Paris, the surprise was general, and the virtuosi began immediately to consider how it could be repeated. It was determined to apply hydrogen gas on this occasion ; and Charles, a celebrated lecturer on natural philosophy, undertook the superintendence of the process. On the 26th of August, 1783, the preparations were complete, and the balloon was trans ported with much ceremony to the Champ de-Mars. On the following day, at five o'clock in the afternoon, the report of a cannon announced to the assembled mul titude that every thing was ready. "The i globe, liberated from its stays shot up wards, to the great surprise of the spec tators, with such rapidity that in two minutes it reaohed the height of 8000 feet. It traversed successively several clouds, by which it was repeatedly ob scured. The violent rain which began to fall at the moment of its ascent did not retard its rapid_progress, and the experi ment was attended with complete success. The satisfaction was so great that even elegantly dressed ladies remained with their eyes intently fixed on the balloon, regardless of the which fell on them in torrents." (Libel. Dictionnaire de Pitysique.) This balloon remained in the atmosphere only three quarters of an. hour ; it fell at a distance of about fifteen miles, when it was discovered that a rent was made in the upper part, through which the gas had escaped.
The first adventurers who had courage to undertake an aerial ascent in a balloon, were Pilatre de Rosier, a young naturalist, and the Marquis d'Arlandes. On the 21st of November, 1788, they took their seats in the basket of a smoke balloon ; and after rising to an elevation of upwards of 8000 feet, descended safely to the earth. The next ascent was made by MM. Charles and Robert in a balloon filled with hydrogen gas, on the 1st of January, 1784. After a flight of a hour and a half they alighted on the meadow of Nesle, about twenty-five miles from Paris, with out the slightest accident. As the bal loon still retained a considerable buoyant force, M. Charles resolved on another ascent alone. It rose to the height of near two miles in about ten minutes- and the aeronaut had the satisfaction of see ing the sun, which had set when he left the earth, again rise above the horizon. After remaining about thirty-five minutes in the air, he descended safely at a dis tance of about nine miles from the spot from which he had risen.